High-Density Soft-Matter Electronics

ABSTRACT

The disclosure describes a soft-matter electronic device having micron-scale features, and methods to fabricate the electronic device. In some embodiments, the device comprises an elastomer mold having microchannels, which are filled with an eutectic alloy to create an electrically conductive element. The microchannels are sealed with a polymer to prevent the alloy from escaping the microchannels. In some embodiments, the alloy is drawn into the microchannels using a micro-transfer printing technique. Additionally, the molds can be created using soft-lithography or other fabrication techniques. The method described herein allows creation of micron-scale circuit features with a line width and spacing that is an order-of-magnitude smaller than those previously demonstrated.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. §119 of ProvisionalSer. No. 62/176,107, filed Feb. 9, 2015, which is incorporated herein byreference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Not applicable.

BACKGROUND OF THE INVENTION

This invention relates to soft-matter electronics. More particularly,this invention relates to soft-matter electronic devices withmicron-scale circuit feature sizes, and methods of fabricating suchelectronic devices.

Soft-matter electronics have a range of applications, from personalcomputing, assistive medical robotics, and other application domainsthat require safe and comfortable human-machine interaction. In oneexample, soft-matter electronics are incorporated into wearableelectronics, which are electronic devices designed to be worn by a userin close contact with their body. To allow body parts to move freely,wearable electronics must be intrinsically soft and stretchable in orderto match the elastic compliance of natural human tissue. In the past,materials such as conductive fabrics and meandering wires have been usedto allow movement. More recently, “soft-matter” electrical wirescomposed entirely of soft elastomers, gels, and fluids have been used.In these soft microfluidic electronics, a fluidic channel of liquidalloy embedded in an elastomer functions as an electrically conductivewire that remains conductive as the surrounding elastomer is stretched.

In some soft-matter electronics, a eutectic Gallium-Indium (EGaIn) metalalloy is used as the conductive material. The EGaIn traces are liquid atroom temperature, and thus, they remain intact and electricallyfunctional as the surrounding elastomer elastically deforms duringstretching and bending. Since the alloy is a liquid at room temperature(above about 5° C. or higher), the alloy is sealed within microchannelsformed in an elastomer to prevent the alloy from escaping.

Soft-matter electronics offer many advantages to enable electronicdevices to be conformable in various applications since they remainmechanically intact and electrically functional under extreme elasticdeformation. For instance, the intrinsic elasticity enables compliancematching with human tissue and allows soft-matter electronics tocomplement metal-coated textiles, wavy circuits, and otherelastically-deformable technologies that can be worn on the skin orimplanted in the body without interfering with natural bodily functions.Fabrication and functionality of several soft-matter electronic deviceshave been demonstrated including antennas, complex circuit components,and strain, force, and pressure sensors.

Previous fabrication methods have included mask stencil lithography,droplet-based transfer microcontact printing (μCP), freeze-casting,laser engraving, and deposition with a motorized capillary. However, theaforementioned patterning techniques have only been used to producecircuits with feature sizes greater than 30 μm. The main limitation ofprevious techniques is that they generally involve injection of liquidalloys under pressure into microchannels or onto target elastomericsurfaces. Creation of single-micron scale structures requires very highpressures that can exceed the elastic modulus of the elastomer and leadto mechanical failure.

Stated differently, soft and stretchable microelectronics withmicron-scale line widths that will enable the circuit density andsophisticated functionality of conventional rigid microelectronics havenot been demonstrated using previously known techniques. Therefore, itwould be advantageous to develop a method that allows fabrication ofsoft-matter electronics with feature sizes approaching those oftraditional rigid microelectronics.

BRIEF SUMMARY OF THE INVENTION

According to embodiments of the present invention are soft-matterelectronics with micron-scale features, and methods to fabricate suchelectronics. In one embodiment, microchannels are created in anelastomer using soft-lithography and replica molding techniques.Subsequently, micro-transfer printing is used to fill a eutectic alloyinto the microchannels, which are then sealed with another layer ofelastomer. The method of the present invention allows fabrication ofmicron-scale circuit features with a line width and spacing that is anorder-of-magnitude smaller than those previously demonstrated.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 illustrates the various steps of fabricating a soft-matterelectronic device according to one embodiment.

FIG. 2 is a stereo microscope image of an array with a eutectic alloydeposited in channels of the array.

FIG. 3A is a stereo microscope image of an array prior to deposition.

FIG. 3B is a stereo image of an array after deposition of the eutecticalloy.

FIGS. 4A-4C are three dimensional atomic force microscope images ofpartially emptied channels having widths of 10 μm, 5 μm, and 2 μm,respectively.

FIGS. 5A-5C are graphs depicting cross-sectional atomic force microscopedata from the filled and emptied sections of the channels (top row) andcross-sectional atomic force microscope profiles of the channels priorto deposition with Gallium-Indium alloy (bottom row), corresponding tochannels having widths of 10 μm, 5 μm, 2 μm, respectively.

FIGS. 6A and 6B are microscope images of wire arrays with line widths of5 μm (FIG. 6A) and 2 μm (FIG. 6B).

FIG. 7 is a graph showing measured values and Ohm's Law-basedpredictions for the resistances of 5 μm and 2 μm wide wires fordifferent number of wires connected in parallel.

FIG. 8 shows microscope images of the interdigital co-planar capacitorcreated through local disconnection of the 5 μm wide wires andassociated capacitance measurements.

FIGS. 9A-9D show the experimental setup for mechanical testing of wiresfabricated according to one embodiment of the present invention (FIG.9A) and stereo-microscope images of the wire array and its interfacingwith the alloy droplets (FIGS. 9B-9D).

FIGS. 10A-10D are magnified microscope images of the 5 μm wide wirearrays at various strain levels.

DETAILED DESCRIPTION OF THE INVENTION

According to embodiments of the present invention is a soft-matterelectronic device 300 and methods of fabricating the electronic device300. Soft-matter electronics maintain conductive functionality whilebeing stretched and deformed. In contrast to conventionalmicroelectronics, these circuits are composed of a soft elastomerpackage comprising microfluidic channels 301 filled with eutectic metalalloy 302, such as a Gallium-Indium alloy (EGaIn). Eutectic metal alloys302, such as EGaIn, are liquid at room temperature, and thus, theyremain intact and electrically functional as the surrounding elastomerelastically deforms during stretching and bending.

In addition to alloys such as EGaIn, the soft matter electronic device300 can use other eutectic alloys such as Galinstan®(Gallium-Indium-Tin), NewMerc®, Indalloy®, and similar alloys of Galliumand Indium, as these alloys offer a non-toxic alternative to mercury andare liquid near room temperature. In addition, these alloys areparticularly attractive for their high electrical conductivity, which isabout 1/20^(th) the conductivity of copper and is orders of magnitudegreater than conductive grease and electrolytic solutions.

The fabrication method, according to one embodiment, uses techniques insoft lithography and micro-transfer printing. Soft lithography is usedto create a master mold or template 201 of the circuit 305, which can beused to create an elastomer mold 303 comprising a plurality ofmicrochannels 301 replicating the circuit 305. Micro-transfer printingis used to transfer the alloy 302 into the microchannels 301 of theelastomer mold 303. While this particular embodiment relies upon softlithography to create the template 201, alternate embodiments usedifferent fabrication techniques.

Referring to FIG. 1, at step 101 an elastomer mold 303 with micron-scaleconcave features (i.e., microchannels 301) is created through elastomerreplica molding. In this embodiment, soft lithography is used to createthe master mold (or template) 201, on which the microchannels 301 arepositive features rising above the surface of the master mold 201. Inturn, the template 201 is used to create the elastomer mold 303, wherethe microchannels 301 are depressions on the surface of the elastomer.In alternative embodiments, other methods can be used to create theelastomer mold 303, including but not limited to direct patterning ofelastomers through focused-ion beam lithography, electron-beamlithography, or micro-scale 3D printing of the elastomeric mold. In thisconfiguration described above, the elastomer mold ispolydimethylsiloxane (PDMS), but other elastomers such as UV-curablepolyurethanes (PU) and derivatives of polyvinyl siloxanes (PVS), amongothers, can be used.

At step 102, the liquid-phase eutectic alloy 302, such as EGain, isdeposited as a thin film across an elastomeric “donor” substrate 202. Inone embodiment, the donor substrate 202 is created by curing anelastomer against a flat silicon wafer. As shown in FIG. 1, the alloy302 is spread with a roller 203. At step 103, the deposited alloy 302 isflattened by compression using a flat elastomeric substrate 204 undernormal force. In one configuration, the film is about a few nanometersto several microns thick. While this particular method describes atwo-step process using a roller 203 and flattening substrate 204,alternative methods that create a substantially uniform film on thesurface of the donor substrate 202 can be used.

Subsequently, enabled by the unique wetting properties of the alloy 302in air, micro-transfer printing is used to fill the microchannels 301with alloy 302 at step 104. During step 104, the mold 303 is pressedonto the donor surface 202, which contains a thin film of the alloy 302on its surface. When the elastomer mold 303 is separated from the donorsubstrate 202 at step 105, the channels 301 remain filled with alloy301. At step 106, the microchannels are sealed with another layer 304 ofelastomer, such as PDMS.

This versatile fabrication technique can be used to pattern a eutecticalloy 302 into any planar network of microfluidic channels 301 that canbe formed on an elastomer mold 303. As an example of this method offabrication, FIG. 2 shows micro-patterned EGaIn over a large (mm²)surface area. In this embodiment, a PDMS mold 303 is patterned witharrays of channels 301 having a nominal depth of 1 μm and length of 1.5mm. Three different channel-width and inter-channel spacing combinationswere included on the mold 303: 10 μm width with 10 μm spacing; 5 μmwidth with 5 μm spacing; and 2 μm width with 1 μm spacing. PDMS is usedas the donor substrate 202 and EGain is used as the alloy 302. Lightintensity maps acquired from optical profilometry of the 2 μm widechannels are shown in FIG. 3A (before EGain deposition) and in FIG. 3B(after EGaIn deposition). In these figures, EGaIn filled channels 301are recognized by higher light intensity (lighter color in the drawing).

In this exemplar embodiment, the elastomer mold 303 is fabricated by atwo-step replica molding process. In the first step, a template 201 iscreated using an AFM step height standard including thermally grownsilicon dioxide features on a silicon substrate (for example, AppNanoSHS-1) as a guide. An AFM step height standard can contain grating witha pitch of 3 μm, for example, making it suitable for defining the singlemicron-wide microchannels 301. Using the step height standard as aguide, a UV-curable polymer (for example, Norland Adhesives NOA-63) isapplied to the standard to create its reverse replica. For this purpose,the liquid polymer precursor is applied on the guide and cured using aUV light source, such as a Black Ray UV-light, 365 nm wavelength) at21.750 mW·cm⁻².

The polymer production mold 201 created from the height standard is thenmolded by a two part PDMS (for example, Slygard 184 Dow Corning, 10:1mass ratio) to produce an elastomer mold 303 containing microchannels301. A PDMS donor substrate 202 is created by a curing two-part PDMSagainst a flat silicon wafer using a larger mass ratio (15:1), which canimprove wettability by EGaIn compared to 10:1 mass ratio. In thisexample, a droplet of EGaIn is then introduced on the donor substrate202 using a syringe and manipulated to form a smooth (40 nm Racharacterized by optical profilometry), thin film. Both the roller 203and the flat elastomer substrate 204 used to spread and flatten theEGaIn film are made of PDMS (10:1 mass ratio).

To accurately place the elastomer mold 303 against the thin film, theelastomer mold 303 is glued to a glass slide and then attached to amotorized vertical stage (ThorLabs MTS/50-Z8, for example) which is usedto establish controllable contact between the mold 303 and the EGaInfilm. The donor substrate 202 is attached to a kinematic mount (ThorLabsK6XS, for example), which enables making angular alignments between themold 303 and donor substrate 202. The EGaIn deposited mold 303 is thensealed through polymerization of the sealing PDMS layer 304 (10:1 massratio) on the mold 303. All PDMS samples are polymerized at 50° C. for 8hours.

FIGS. 4A, 4B, and 4C show atomic force microscopy (AFM) images ofdifferent sections of the soft matter electronic device 300 created bythe foregoing example fabrication method. The deposited EGain patternsin the device 300 were locally and selectively damaged prior to sealingusing a sharp tungsten probe with sub-micron tip radii. The regions thatinclude the interface between the damaged and undamaged EGaIn patternsare shown in FIGS. 4A-4C.

Cross-sectional profiles of the channels 301 prior to EGaIn depositionare shown in the bottom row of FIGS. 5A-5C for line widths of 10 μm, 5μm, and 2 μm, respectively. The top row of FIGS. 5A-5C show the profileof alloy 302 deposited within the microchannels 301. As shown, thechannels 301 are partially filled, forming “wires” of EGaIn along thelength of the channels 301. Furthermore, a thin residual layer of EGaIncan be observed outside the channels 301. This residual layer causesdeformation of the sidewalls of the channels 301, particularly for thearrays with 1 μm spacing (FIG. 5C). For channel arrays with largerspacing, periodic textures were observed on the sidewalls. The AFMmeasurements suggest that the thickness of the residual layer isapproximately 10 nm.

Referring to FIGS. 6A and 6B, conductivity measurements were performedon 5 μm and 2 μm wide wires to study the electrical characteristics ofthe circuit 305 created by the patterned alloy 302. As shown in FIGS.6A-6B, tungsten probes were inserted into two large droplets of EGaInadministered at the end of the wires prior to sealing. During themeasurements, the inherent resistance of the measurement loop(resistance of the probes, contact resistance between the probes and thedroplets etc.) were quantified and subtracted from the measuredresistance. To correlate the resistance values with the number of EGaInwires, a sequential approach was used: the resistance was first measuredfor the largest number of wires. For each of the following measurements,one wire was severed using the tungsten probe before each measurement.Approximately six orders of magnitude increase in the measuredresistance was observed after the disconnecting all the wires for bothof the cases.

To compare the conductivity of the created circuits with the bulkconductivity of EGaIn, the measured resistance values were compared withvalues that were predicted using Ohm's Law. The predicted resistancevalues were calculated as

$\begin{matrix}{{R = \left( {\sum\limits_{i = 1}^{n}\; \frac{1}{R_{i}}} \right)^{- 1}},} & (1)\end{matrix}$

where n is the number of the wires connected in parallel and R_(i) isthe individual resistances of the measured wires given by

$\begin{matrix}{R_{i} = {\rho {\int_{0}^{L}\ {\frac{x}{A_{i}(x)}.}}}} & (2)\end{matrix}$

Here, ρ is the bulk resistivity of EGaIn (29.4×10⁻⁶ Ωcm), x is thecoordinate along the length of a wires, L is the total length of thewire, and A_(i)(x) is the cross-sectional area of the wire at anarbitrary location along the length of the wire. To determine A_(i)(x),optical profilometry measurements of the wires both before and afterthey were disconnected were performed. Assuming that the channels 301were filled entirely below the measured top surface, the cross-sectionalarea at a given location was calculated by integrating the differencebetween the filled and emptied cross-sections of the channels 301. Asshown in FIG. 7, the measured and predicted resistance values show astrong agreement for both channel 301 sizes.

A number of critical conclusions can be drawn from the results of theconductivity testing and the agreement between the measured values ofwire resistances and the predictions based on Ohm's Law: (1) themicrochannels 301 exhibit the same level of electrical conductivity asbulk EGaIn alloy 302; (2) a strong agreement between the predicted andmeasured resistances for multiple parallel microchannels 301 and, themultiple orders of magnitude increase in resistance after disconnectingall of the wires formed by the microchannels 301 indicate that the wiresare not shorting across the inter-wire spacing, which can be as small as1 μm with the method of the present invention (the results signifiesthat the residual layer between the channels 301 consists ofnon-conductive oxides of the alloy 302); and (3) the channels 301 on theelastomer mold 303 are completely filled below the surface of thedeposited alloy 302.

FIG. 8 shows an example of a functional circuit element 305—a co-planecapacitor—that can be created using the method of the present invention.In this example, a capacitor 400 comprises 5 μm wide microchannels 301with 5 μm spacing, which are selectively disconnected to form a combpattern with a total of ten fingers 401. The capacitance measurementacross the capacitor 400 is 0.2 pF, as shown in the top image of FIG. 8.To quantify any parasitic capacitive effect (e.g., those due to theapplied droplets or the residual layer), another capacitance measurementwas performed after disconnecting all the fingers 401, as shown in thebottom image of FIG. 8 as 0.07 pF. Assuming a parallel electricalconnection between the co-planar capacitance of the created circuit 305and the parasitic capacitance, the effective capacitance of theEGaIn-based capacitor 400 is 0.13 pF.

The total surface area within which the measured capacitance is achievedis approximately 2×10⁻⁸ m², yielding a density of capacitance over theplanar area as high as 6.5 μF/m². This is significantly higher than the˜10 nF/m² capacitance density previously achieved with EGaIn-basedsoft-matter electronics. Stated differently, the method and device 300of the present invention exhibits a 650× increase in capacitance densitycompared to EGaIn-based circuits previously produced with needleinjection techniques. As this example illustrates, the smaller channel301 width allows the creation of circuits 305 with greater functionalitycompared to devices created with prior methods. As a person having skillin the art will appreciate, feature size can be a critical factor indesigning a device. For example, smaller feature size reduces thedistance signals propagate through the circuit 305, lowers parasiticcapacitance, and can allow the use of lower voltage.

A capacitor is just one example of a circuit 305 that can be createdusing the method of the present invention. The method can be used tocreate additional circuits 305, such as a strain gauge, pressure gauge,force gauge, antenna, connective wires, or other devices utilizingelectromechanical functionalities.

One of the key requirements for soft-matter electronics is for them tomaintain their electronic functionality during elastic deformation. FIG.9A depicts an axial loading test on a sample soft-matter electronicdevice 300 having 5 μm wide microchannels 301. For this purpose, thesealed elastomeric mold 303 was clamped at its two ends in a verticalconfiguration as shown in FIG. 9A. In this configuration, the bottomclamp was kept stationary, whereas the top clamp was moved vertically,thereby stretching the mold 303. During stretching, the EGaIn-filledmicrochannels 301 were observed using a microscope through thetransparent sealing layer 304. The axial force on the top clamp wasmeasured using a force gauge. To enable conductivity measurements duringthe mechanical testing, alloy droplets were administered at both ends ofthe microchannels 301 prior to sealing (as shown in FIG. 9B), and copperwires were embedded inside the droplets and the sealing layer 304. Themagnified microscope images of the EGaIn-filled microchannels 301 atvarious axial strain levels along with the measured axial force andresistance values are shown in FIGS. 9C-9D and FIGS. 10A-10D. As shown,the test was extended up to 40% axial strain. No visual degradation ofthe integrity of the EGain-filled microchannels 301 is observed up to40% axial strain.

As shown in the figures, the resistance across the microchannels 301 is10.4Ω with zero strain (FIG. 9D), 10.35Ω at 10% strain (FIG. 10A), 10.4Ωat 20% strain (FIG. 10B), 10.9Ω at 30% strain (FIG. 10C), and 12Ω at 40%strain (FIG. 10D). In summary, the resistance at 40% strain increasedless than 20% of the unstrained value. The resistance values indicatedabove are not compensated for the resistance of the measurement loop.

The resistance corresponding to the wires should be approximately 3Ω andthus, the measurement loop resistance should be approximately 7Ω.Considering the mechanical properties of PDMS (Poisson's ratio˜0.5), itis expected for the EGain-filled microchannels 301 resistance toincrease quadratically with the amount of stretch (1+strain).Accordingly, a measured resistance change of less than 2Ω is reasonable.After the testing was complete and the mold 303 was taken out of theclamps, the resistance across the wires was measured to be 10.6Ωindicating that no apparent loss in conductivity occurred uponrelaxation of the strain.

While the disclosure has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modification can be made therein withoutdeparting from the spirit and scope of the embodiments. Thus, it isintended that the present disclosure cover the modifications andvariations of this disclosure provided they come within the scope of theappended claims and their equivalents.

What is claimed is:
 1. A method of creating a micron-scale soft-matterelectronic device, comprising: creating an elastomer mold containing aplurality of microchannels forming an electronic circuit; depositing athin film of eutectic alloy on a donor surface; mating the elastomermold onto the donor surface, wherein the eutectic alloy is drawn intothe plurality of microchannels; removing the elastomer mold from thedonor surface, wherein the eutectic alloy remains in the plurality ofmicrochannels; sealing the plurality of microchannels with an elastomer.2. The method of claim 1, wherein creating the elastomer mold comprises:creating a template using a fabrication technique; molding the elastomermold from the template.
 3. The method of claim 1, wherein depositing athin film of eutectic alloy on a donor surface comprises: applying thealloy to the donor surface; spreading the alloy using a roller; levelingthe alloy using a substrate, wherein a substantially uniform filmremains on the donor surface.
 4. The method of claim 1, wherein theelastomeric mold is created from polydimethylsiloxane.
 5. The method ofclaim 1, wherein the donor surface is created from polydimethylsiloxane.6. The method of claim 5, wherein the donor surface has a mass ratio ofabout 15:1.
 7. The method of claim 1, wherein the eutectic alloy isliquid above 5° C.
 8. The method of claim 2, wherein the fabricationtechnique is soft lithography.
 9. The method of claim 1, wherein theplurality of microchannels have a spacing of 1 μm to 10 μm.
 10. Asoft-matter electronic device, comprising: an elastomer mold containinga plurality of microchannels forming an electronic circuit, wherein theplurality of microchannels have a width of less than 30 μm; a eutecticalloy disposed within the plurality of microchannels; and a barriercovering the microchannels.
 11. The soft-matter electronic device ofclaim 10, wherein a spacing between adjacent microchannels is about 1 μmto 10 μm.
 12. The soft-matter electronic device of claim 10, wherein thespacing between adjacent microchannels is about 1 μm.
 13. Thesoft-matter electronic device of claim 10, wherein the plurality ofmicrochannels have a width from about 2 μm to 30 μm.
 13. The soft-matterelectronic device of claim 10, wherein adjacent microchannels areseparated by a layer of non-conductive oxide of the eutectic alloy. 14.The soft-matter electronic device of claim 10, wherein the electroniccircuit comprises a capacitor having a plurality of fingers forming acomb pattern.
 15. The soft-matter electronic device of claim 14, whereina density of capacitance of the capacitor is at least 6.5 μF/m².
 16. Thesoft-matter electronic device of claim 14, wherein a density ofcapacitance of the capacitor is greater than 10 nF/m².
 17. Thesoft-matter electronic device of claim 10, wherein the electroniccircuit remains conductive when the elastomer mold is stretched by up toabout 40% of an initial state.
 18. The soft-matter electronic device ofclaim 10, wherein the elastomer mold is polydimethylsiloxane.
 19. Thesoft-matter electronic device of claim 10, wherein the eutectic alloycomprises an alloy of Gallium and Indium.
 20. A product by the method ofclaim 9.